Balk's

The 1st Baseman (or the 3rd Baseman) must be completely in fair territory while the Pitcher is on the rubber and in possession of the ball.

Whether or not it is called a balk seems to be league specific. Some leagues merely force the player to step into fair territory, others will call it a fielder's balk.

The most extreme case which I have ever seen was a ruling that a fielder's balk is a delayed call. The call is made after the pitch is made and allows the team at bat with the best of situations. If the batter doubles, despite the balk the ball remains in play and he is allowed that hit. However, if the batter swung and missed, the balk would be enforced.

Regardless, check your league's rule book. You should find the applicable ruling in the same area which notes that the Catcher must remain in the "box" until the ball leaves the Pitcher's hand.